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There are many different carbon allotropes. Some are bulk materials, such as diamond or graphite, whereas others are nanostructures materials – such as carbon nanotubes (CNTs) and graphene. All carbon allotropes are known to be relatively good at conducting heat, with some being more efficient than others. Here, we’re going to look at how CNTs and the other nanostructured carbon allotropes can be used to transport heat.
When many people think of nanomaterials such as CNTs and graphene, more emphasis is put on their structural and/or electronic conductivity properties. The only time that the thermal properties of these materials become a consideration is when they are being investigated for thermally protective coatings.
There has been more interest of late regarding the properties of various materials that can be formulated into these types of coatings due to the increase in thermal energy being emitted in modern-day electronic devices – something which is ever increasing as technology gets more efficient and powerful. CNTs, graphene and other nanocarbon materials are now an option to remove excess heat in many electronic devices, but how do they transport the heat away?
Heat transport is a phenomenon that is governed by the atomic structure of a material. Heat is carried through materials by acoustic phonons and the phonon can pass through with minimal to low impedance that facilitates a high thermal conductivity. These phonons are essentially a (quantum mechanical) molecular vibration and the easier that this vibration can transfer across the atomic lattice, the greater the thermal conductivity is. However, this is just the tip of the iceberg in terms of thermal transport, as many different types can change how effectively the material transports heat.
Bulk Carbon Allotropes
It should be noted that bulk carbon allotropes and nanostructured carbon allotropes transport heat in different ways and the transport of phonons in nanostructured materials is often a short path due to scattering at the edges of the nanomaterial.
The allotropes, which fall into the category of bulk carbon allotropes, include diamond, graphite and amorphous carbon. Diamond is known to have high heat transport due to its regular sp3 ordered array of carbon atoms. But, because it has a very rigid structure, diamond also has low thermal expansion, making it a good (but costly) transporter of heat.
The thermal conductivity in graphite can vary depending on the quality and the number of layers, but it is a relatively good conductor of in-plane heat. The transport properties for amorphous carbon again vary depending on the quality and atomic structure, and the properties can range from a very bad thermal conductor to a satisfactory conductor.
Carbon Nanotubes (CNTs) and Graphene
The heat transport properties of nanostructured carbons can significantly vary depending on the type and structure. In many nanostructured carbons, the thermal conductivity is limited by the scattering that occurs at the edge planes or grain boundaries within polycrystalline and amorphous nanomaterials. This is often the case for more disordered carbon nanostructures, such as those which rely on sp3 networks and have varying grain sizes.
It’s a different story for the more ordered and purer carbon nanomaterials, such as CNTs and graphene where the thermal conductivity is governed by the intrinsic properties of the sp2 lattice. The thermal conductivities are not quite as high in CNTs compared to graphene because of the curvature of the CNT, as it leads to the phonons being quantized differently and slightly suppresses the thermal conductivity compared to a flat graphene sheet. However, both graphene and CNTs show a higher thermal conductivity than many other materials, including other carbon allotropes (nanostructured and bulk).
In terms of CNTs, single-walled CNTs (SWCNTs) are known to have slightly better thermal transport properties than multi-walled CNTs (MWCNTs), but both types show greater thermal transport properties than graphite and diamond.
The better thermal transport values in SWCNTs are thought to be due to the interactions that arise between phonons and electrons between the layers in MWCNTs - which can suppress the thermal conductivity properties of the CNT. In general, the more walls that a CNT has, the lower the thermal conductivity, as there are more of these interactions. The strong carbon network of CNTs also prevents them from expanding under high heat.
Alongside a very high electrical conductivity, graphene also has very efficient heat transport properties, which are again much better than graphite and diamond, as well having higher transport values than CNTs.
The values in graphene can vary depending on how many layers of graphene are present and what the dimensions of the layer are. Depending on the size, the thermal conductivity can be suppressed by short or disordered (rough) edges. The larger the lateral size of the graphene layer, the greater the thermal conductivity.
While graphene has a high in-plane thermal conductivity, it has poor cross-plane conductivity, so the better thermal conductivity properties are found in graphene materials with fewer layer numbers. The presence of a Brillouin zone (BZ) in graphene means that the phonons can have any frequency, and this is also a factor in why graphene can have a very high thermal conductivity.
References and Further Reading
“Thermal Properties of Graphene, Carbon Nanotubes and Nanostructured Carbon Materials”- Balandin A. A., Nature Materials, 2011, DOI: 10.1038/nmat3064